I. Technical Field
This invention pertains to wireless telecommunications, and particular to structure and operation of a wireless receiver.
II. Related Art and Other Considerations
In a typical cellular radio system, wireless user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer−included, or car-mounted mobile devices which communicate voice and/or data with radio access network. Alternatively, the wireless user equipment units can be fixed wireless devices, e.g., fixed cellular devices/terminals which are part of a wireless local loop or the like.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks. The core network has two service domains, with an RNC having an interface to both of these domains.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UMTS is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a radio access network providing wideband code division multiple access (WCDMA) to user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM-based radio access network technologies.
In the forthcoming evolution of the mobile cellular standards (like GSM and WCDMA) new modulation techniques (like Orthogonal-Frequency-Division Multiplexing (OFDM)) are likely to occur. Furthermore, in order to have a smooth migration of the “old” cellular systems to the new high capacity high data rate system in the existing radio spectrum, the new system has to be able to operate on a flexible bandwidth. A proposal for such a new flexible cellular system is 3G Long Term Evolution (3G LTE). 3G LTE can be seen as an evolution of the 3G WCDMA standard. That system will use OFDM as a multiple access technique (called OFDMA) in the downlink, but the current assumption is that SC-FDMA (Singe-carrier Frequency Division Multiple Access) will be used in the uplink (UL).
As the current LTE concept stands, the available bandwidth in the SC-FDMA uplink is divided into resource units (RUs) of 180 kHz each. The total occupied bandwidth (BW) of the system varies between 1.25 MHz up to 20 MHz, and the number of RUs varies accordingly. Each user equipment unit (UE), which can be, e.g., mobile station, mobile terminal, wireless station, is allocated an integer number of consecutive RUs, located somewhere in the frequency band. Which RUs, and how many RUs are allocated to each UE is decided by a scheduler in a base station (e.g., in a NodeB), and depends, e.g., on propagation channel conditions and the amount of data to be transmitted. This is thus dynamic, and can change very rapidly.
A SC-FDMA (Singe-carrier Frequency Division Multiple Access) system, similar to a OFDMA system, has a useful property that the frequencies allocated to different users are orthogonal to each other, and can thus be separated without interfering each other. In practice, however, there are several factors that can destroy the orthogonality, leading to inter-frequency interference. One such factor is the spectral impurity that is inevitable in any practical radio transmitter. For example, an image problem can be caused by gain and phase quadrature imbalance, henceforth denoted IQ imbalance, in the transceiver.
IQ imbalance is illustrated in FIG. 1, which depicts an emitted spectrum from a UE transmitting on the 20 “leftmost” resource units (RUs) in a 20 MHz system, occupying frequencies from −9 to −5.4 MHz, where 0 MHz indicates the center of the 20 MHz carrier. Due to imperfections in the (analog) circuitry, an image of the desired signal will be transmitted, on frequencies from 5.4 MHz to 9 MHz in FIG. 1. What frequencies the image is transmitted on depends on where the frequency of the local oscillator is located. In FIG. 1 it is assumed that the local oscillator is located at the center of the band, i.e. at 0 MHz. If the local oscillator would instead been located in the middle of the desired signal, i.e. at −7.2 MHz, the image would have landed onto the desired signal itself, and would not have been visible in the spectrum plot.
Different techniques for suppressing the size of the image can be applied to the transmitter. However, since the IQ imbalance varies with individual components, it is difficult to guarantee extensive image rejection for every single UE.
Since the image generated by a transmitter such as a UE is not projected onto the frequencies allocated to the UE by a scheduler of a base station node (e.g., NodeB), the reception in the NodeB of the signal on the allocated frequency will not be compromised. However, the UE having the signal shown in FIG. 1 will cause interference for UEs transmitting at the image frequencies. If the UEs transmitting on those image frequencies are received at much lower power in the NodeB, the image from the first UE will indeed cause a severe signal to noise ratio (SNR) degradation at the NodeB receiver for these UEs, as is illustrated in FIG. 2. FIG. 2 shows by line UE 1 the received signal of a stronger of two transmitters, and by line UE 2 the received signal of the weaker of the two transmitters, the weaker transmitter operating at a mirror frequency of the operating frequency of the stronger transmitter. As shown in FIG. 2, there is a severe signal to noise ratio (SNR) degradation at the NodeB receiver for the signal received from the weaker transmitter (UE 2).
The effect illustrated by FIG. 2 may be mitigated by the use of UL power control (open or closed loop) by aiming at making the signals from the different UEs being received at the NodeB with equal power. However, such power control will never be perfect due to phenomena and occurrences such as fast fading, measurement and signaling errors, terminals already transmitting at maximum or minimum power etc. Thus, there will always be situations where the received power from different UEs varies significantly.
The problem is similar for an OFDMA system. Moreover, in an OFDMA system, instead of having only one “high” spectrum allocation (desired signal) as in FIG. 1 and one “low” spectrum allocation (image), it is also possible to have several high peaks and several lower images.
There are different techniques which attempt, at least in part, to address the image protection problem.
A first technique involves fast closed-loop power control. If the power control loop is sufficiently fast, the difference in received power between different UEs can be made smaller by following the fast fading. However, the power control will still not be perfect, for reasons mentioned above. Therefore, this first technique is not a sufficient solution to the problem.
A second technique requires improved image rejection at the transmitter. However, very high image rejection on each individual UE may require extensive trimming, which is time consuming and costly. Adaptive solutions are possible, but are difficult since it requires suppression of a weak signal within a large one, and also since the causes of the IQ imbalance appear relatively late in the transmission (TX) chain.
A third technique involves fast change of local oscillator frequency. If the local oscillator is always located in the middle of the desired transmitted signal, the image will be projected onto the desired signal. Image projection on a given signal can be avoided by changing the local oscillator frequency. However, there are several implementation issues with changing the local oscillator frequency. One issue concerns the fact of the if resource unit (RU) allocation switches rapidly, the local oscillator needs to adjust to the new frequency within fractions of the cyclic prefix. This means less than 1 μs, which is orders of magnitude from what is currently feasible in a low-cost radio.
A fourth technique involves interference-aware resource allocation. It is possible for the NodeB to schedule users in the frequency domain to match the induced disturbance with the received powers and the required SNR for different UEs. This option may however not always be feasible or desirable.
What is needed, therefore, and an object of the present invention, are better apparatus, methods, and techniques for compensating for an image signal at a wireless receiver.